CS1 Lecture 15 Feb. 19, 2018

Transcription

1 CS1 Lecture 15 Feb. 19, 2018 HW4 due Wed. 2/21, 5pm (changed from original 9am so people in Wed. disc. sections can get help) Q2: find *any* solution. Don t try to find the best/optimal solution or all of them Q3: although you cannot use a loop in Q2, you may use a loop in Q3 (but function must also be recursive) Exam 1, Thursday evening, 2/22, 6:30-8:00pm, MacBride Auditorium You must bring ID No points in this week s discussion sections. Not required. Consider them just extra office hours to get help with HW4 or other problems. Wednesday: exam review, including sample questions

2 Discussion of HW4 Q2 ideas Suppose we want to make 87 with coins of 3, 5, 8 Check if you can make 84 with 3, 5, 8 If yes, answer for 87 is a value 3 coin plus coins that make 84 (if check above failed) check if you can make 82 with 3, 5, 8 If yes, answer for 87 is a value 5 coin plus coins that make 82 (if check above failed) check if you can make 79 with 3, 5, 8. If yes, answer for 87 is a value 8 coin plus coins that make 79 If all three checks fail, then 87 cannot be made with 3, 5, 8 Important: each check if is a simple recursive call!

3 Last time Started recursion (end of Ch 5) Today A few words on Exam 1 More recursion examples

4 Recursion (end of Ch 5) Very important and useful concept Not just for programming, but math and everyday life, nature, etc. Has undeserved reputation among some people: recursion is bad recursive programs are inefficient Yes, one can write very bad recursive programs but this is true of nonrecursive programs as well. And recursion can be super useful.

5 Recursion Recursive function: function that contains within its definition calls to itself Consider math s factorial. E.g. 3! = 3 * 2 * 1 You might be used to definition like: n! = n * (n-1) * * 2 * 1 But more precise mathematical definition of factorial function is: factorial(1) = 1 factorial(n) = n * factorial (n-1), for all n > 1 Programming-wise, can very directly translate recursive mathematical definitions into code: def factorial(n): if (n == 1): return 1 else: return n * factorial (n 1) DON T let the function call, factorial(n-1), scare you. It s just a function call. If you draw stack frames like we did in earlier lectures, it all works out fine. DO need to think carefully when writing/analyzing recursive programs though

6 Important rules for recursive functions When writing a recursive function: MUST have base case(s), situations when code does not make recursive call. MUST ensure that recursive calls make progress toward base cases. I.e. you need to convince yourself that recursive call is closer to base case than the original problem you are working on SHOULD ensure you don t unnecessarily repeat work. Ignoring this contributes to recursion s bad reputation. E.g. direct recursive implementation of Fibonacci is extremely and unnecessarily inefficient

7 Ch3: Stack frames def countdown(n): if n == 0: print( Blast off! ) else: print(n) countdown(n-1) >>> n = 100 >>> y = 2 >>> countdown(y) 2 countdown: n 0 countdown: n 1 countdown: n 2 1 Blast Off! _main_: n 100 y 2

8 Recursion examples printlist, printlistreverse sumlistitems ispalindrome wasitacaroracatisaw is a palindrome sitonapotatopanotis is a palindrome Fibonacci sequence: 1, 1, 2, 3, 5, 8, fib(0) = 1, fib(1) = 1 fib(n) = fib(n-1) + fib(n-2), for n > 1 flatten a list E.g. [[[[[3,[2,4]]], 0], [ a ]], 23] -> [3,2,4,0, a, 23] generate a number sequence Towers of Hanoi problem lec14.py, lec15.py ToHcomplete.py

9 Towers of Hanoi solution Rods/towers A, B, C. Goal is to move disks from A to C, one at a time, never allowing larger disk to be on top of smaller disk Algorithm: Move n-1 disks from A -> B (by this algorithm!) Move final disk from A -> C Move n-1 disks from B -> C (by this algorithm!) ToHcomplete.py

10 Q3 of HW4 Definition of similar: Two items similar if types equal, and If they are lists, they are equal length and their corresponding items are similar Function outline def aresimilar(item1, item2): # check types # if types not equal return? # if types equal and items are not lists return? # if items are lists of same length # loop over items of lists, checking similarity # (recursively!) of corresponding items # if some pair is not similar return False # if you complete loop without returning False then all # corresponding pairs are similar so you can return True

11 Next Time Review for exam, and sample problems